Known as an indirect method for current sensing on a switch or transistor is the measuring of the forward voltage of the switch. Since the forward voltage in the Ohmic range supplies a voltage that is proportional to the current, it is possible to ascertain the current with the aid of the forward resistance.
Furthermore, in convention methods for current sensing, a current-sensing circuit is switched in parallel with the load circuit so that the current in the load circuit can be ascertained. To do so, a current-sensing transistor may be integrated on the same chip parallel to the load transistor. Both the drain terminals and the gate terminals of the two transistors are connected to each other. To ascertain the load current, the voltage drop at the current-sensing resistance is sensed.
This has the disadvantage that batch variations or non-linearities of the resistances of the current sensing transistor directly affect the current dividing ratio and have a direct influence on the voltage at the current-sensing resistance.
Furthermore, conventionally, current sensing is carried out with the aid of analog-to-digital converters. In order to achieve an accurate measurement, a precise measuring resistor must be used. Because of the linearity of the measuring resistor, an adapted evaluation circuit featuring a high dynamic range is able to detect currents.
In this context it is disadvantageous that the evaluation circuit restricts the measuring range to small currents, such as on account of offset voltages.
An objective of the present invention is to detect small currents with high precision.